memcg: coalesce uncharge during unmap/truncate

In massive parallel enviroment, res_counter can be a performance
bottleneck.  One strong techinque to reduce lock contention is reducing
calls by coalescing some amount of calls into one.

Considering charge/uncharge chatacteristic,
	- charge is done one by one via demand-paging.
	- uncharge is done by
		- in chunk at munmap, truncate, exit, execve...
		- one by one via vmscan/paging.

It seems we have a chance to coalesce uncharges for improving scalability
at unmap/truncation.

This patch is a for coalescing uncharge.  For avoiding scattering memcg's
structure to functions under /mm, this patch adds memcg batch uncharge
information to the task.  A reason for per-task batching is for making use
of caller's context information.  We do batched uncharge (deleyed
uncharge) when truncation/unmap occurs but do direct uncharge when
uncharge is called by memory reclaim (vmscan.c).

The degree of coalescing depends on callers
  - at invalidate/trucate... pagevec size
  - at unmap ....ZAP_BLOCK_SIZE
(memory itself will be freed in this degree.)
Then, we'll not coalescing too much.

On x86-64 8cpu server, I tested overheads of memcg at page fault by
running a program which does map/fault/unmap in a loop. Running
a task per a cpu by taskset and see sum of the number of page faults
in 60secs.

[without memcg config]
  40156968  page-faults              #      0.085 M/sec   ( +-   0.046% )
  27.67 cache-miss/faults
[root cgroup]
  36659599  page-faults              #      0.077 M/sec   ( +-   0.247% )
  31.58 miss/faults
[in a child cgroup]
  18444157  page-faults              #      0.039 M/sec   ( +-   0.133% )
  69.96 miss/faults
[child with this patch]
  27133719  page-faults              #      0.057 M/sec   ( +-   0.155% )
  47.16 miss/faults

We can see some amounts of improvement.
(root cgroup doesn't affected by this patch)
Another patch for "charge" will follow this and above will be improved more.

Changelog(since 2009/10/02):
 - renamed filed of memcg_batch (as pages to bytes, memsw to memsw_bytes)
 - some clean up and commentary/description updates.
 - added initialize code to copy_process(). (possible bug fix)

Changelog(old):
 - fixed !CONFIG_MEM_CGROUP case.
 - rebased onto the latest mmotm + softlimit fix patches.
 - unified patch for callers
 - added commetns.
 - make ->do_batch as bool.
 - removed css_get() at el. We don't need it.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 90 insertions, 6 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 7b5b108c1c6b..a730c91b8e69 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1827,6 +1827,50 @@ void mem_cgroup_cancel_charge_swapin(struct mem_cgroup *mem)
         css_put(&mem->css);
 }
 
+static void
+__do_uncharge(struct mem_cgroup *mem, const enum charge_type ctype)
+{
+        struct memcg_batch_info *batch = NULL;
+        bool uncharge_memsw = true;
+        /* If swapout, usage of swap doesn't decrease */
+        if (!do_swap_account || ctype == MEM_CGROUP_CHARGE_TYPE_SWAPOUT)
+                uncharge_memsw = false;
+        /*
+         * do_batch > 0 when unmapping pages or inode invalidate/truncate.
+         * In those cases, all pages freed continously can be expected to be in
+         * the same cgroup and we have chance to coalesce uncharges.
+         * But we do uncharge one by one if this is killed by OOM(TIF_MEMDIE)
+         * because we want to do uncharge as soon as possible.
+         */
+        if (!current->memcg_batch.do_batch || test_thread_flag(TIF_MEMDIE))
+                goto direct_uncharge;
+
+        batch = &current->memcg_batch;
+        /*
+         * In usual, we do css_get() when we remember memcg pointer.
+         * But in this case, we keep res->usage until end of a series of
+         * uncharges. Then, it's ok to ignore memcg's refcnt.
+         */
+        if (!batch->memcg)
+                batch->memcg = mem;
+        /*
+         * In typical case, batch->memcg == mem. This means we can
+         * merge a series of uncharges to an uncharge of res_counter.
+         * If not, we uncharge res_counter ony by one.
+         */
+        if (batch->memcg != mem)
+                goto direct_uncharge;
+        /* remember freed charge and uncharge it later */
+        batch->bytes += PAGE_SIZE;
+        if (uncharge_memsw)
+                batch->memsw_bytes += PAGE_SIZE;
+        return;
+direct_uncharge:
+        res_counter_uncharge(&mem->res, PAGE_SIZE);
+        if (uncharge_memsw)
+                res_counter_uncharge(&mem->memsw, PAGE_SIZE);
+        return;
+}
 
 /*
  * uncharge if !page_mapped(page)
@@ -1875,12 +1919,8 @@ __mem_cgroup_uncharge_common(struct page *page, enum charge_type ctype)
                 break;
         }
 
-        if (!mem_cgroup_is_root(mem)) {
-                res_counter_uncharge(&mem->res, PAGE_SIZE);
-                if (do_swap_account &&
-                                (ctype != MEM_CGROUP_CHARGE_TYPE_SWAPOUT))
-                        res_counter_uncharge(&mem->memsw, PAGE_SIZE);
-        }
+        if (!mem_cgroup_is_root(mem))
+                __do_uncharge(mem, ctype);
         if (ctype == MEM_CGROUP_CHARGE_TYPE_SWAPOUT)
                 mem_cgroup_swap_statistics(mem, true);
         mem_cgroup_charge_statistics(mem, pc, false);
@@ -1926,6 +1966,50 @@ void mem_cgroup_uncharge_cache_page(struct page *page)
         __mem_cgroup_uncharge_common(page, MEM_CGROUP_CHARGE_TYPE_CACHE);
 }
 
+/*
+ * Batch_start/batch_end is called in unmap_page_range/invlidate/trucate.
+ * In that cases, pages are freed continuously and we can expect pages
+ * are in the same memcg. All these calls itself limits the number of
+ * pages freed at once, then uncharge_start/end() is called properly.
+ * This may be called prural(2) times in a context,
+ */
+
+void mem_cgroup_uncharge_start(void)
+{
+        current->memcg_batch.do_batch++;
+        /* We can do nest. */
+        if (current->memcg_batch.do_batch == 1) {
+                current->memcg_batch.memcg = NULL;
+                current->memcg_batch.bytes = 0;
+                current->memcg_batch.memsw_bytes = 0;
+        }
+}
+
+void mem_cgroup_uncharge_end(void)
+{
+        struct memcg_batch_info *batch = &current->memcg_batch;
+
+        if (!batch->do_batch)
+                return;
+
+        batch->do_batch--;
+        if (batch->do_batch) /* If stacked, do nothing. */
+                return;
+
+        if (!batch->memcg)
+                return;
+        /*
+         * This "batch->memcg" is valid without any css_get/put etc...
+         * bacause we hide charges behind us.
+         */
+        if (batch->bytes)
+                res_counter_uncharge(&batch->memcg->res, batch->bytes);
+        if (batch->memsw_bytes)
+                res_counter_uncharge(&batch->memcg->memsw, batch->memsw_bytes);
+        /* forget this pointer (for sanity check) */
+        batch->memcg = NULL;
+}
+
 #ifdef CONFIG_SWAP
 /*
  * called after __delete_from_swap_cache() and drop "page" account.